A commonly employed signalling technique in present day telecommunication systems involves multifrequency pulsing. Current multifrequency pulsing systems, such as those employing R2 and No. 5 CCITT-defined signals, utilize various combinations of two-out-of-six basic tone frequencies in the voice band. For example, No. 5 CCITT-defined signals consist of six tone frequencies from 700 to 1700 Hz spaced 200 Hz apart. These six frequencies may be combined in pairs to create up to fifteen dual tone or dual frequency combinations. Various combinations of the tone pairs may represent transmitted digits, zero through nine, so that ten digits and five other symbols or signals may be represented by the fifteen dual tone pairs. Since the tone pairs occupy the normal voice band, the signals are normally transmitted over regular talking channels.
Concomitant with the utilization of dual tone multifrequency (DTMF) signalling systems has been the implementation of various techniques for detecting the tone pulses. These techniques include both analog and digital detection schemes through which tones are filtered and subjected to a comparison-evaluation process. For example, there are a number of tone detectors which contain respective analog filters for each of the frequencies of interest and a common gain control coupling arrangement for adjusting the level of the inputs to the respective filters or the levels in the filter channels themselves. Such detectors are undesirably sensitive to noise since they effectively respond to the instantaneous values of their inputs. The U.S. Pat. Nos. to Cowpland 3,795,775, Hanson 3,812,432, and Alaily 3,875,347 contain illustrative descriptions of such detectors. Another type of detector is that which effectively operates as a zero-crossing detector, typically configured in a digital fashion to count repetitive series of clock pulses over a set of timing intervals and indicating detection of a tone if the clock/count corresponds to a synchronization code reference. Reference may be had to the U.S. Pat. Nos. to Pitroda 3,710,028, Hammad 4,016,371, Beeman et al 3,760,269, or Friend 3,537,001 for exemplary descriptions of such detector arrangements. Similarly, the U.S. Pat. to Sharp et al No. 4,021,653 describes a tone detector filter arrangement that is configured substantially of all digital components.
A further type of system is a hybrid configuration containing both channel filters and digital processing components, associated with high frequency tones and with a separate set of low frequency tones. These latter systems attempt to achieve a more accurate identification of valid tone pairs by employing persistence and tone code comparison criteria. This latter approach is considered to provide a technique that is more accurate and less subject to the influence of noise that may be in the form of signal or component variations. Examples of such systems are described in the U.S. Pat. to Laoteppitaks et al Nos. 4,016,370 and Ullakko 3,912,869. Unfortunately, these patented detector arrangements do not process the signals according to the individual frequencies employed in the DTMF system, but segregate an incoming signal into respective high and low frequency channels for processing. As a result, continuous monitoring of each of the tone channels employed in the system is not possible, thereby reducing the accuracy of the tone pair identification. In addition, flexibility in the choice of tone pairs is limited by preassigned upper and lower channel separation.